The Shanti Mantras or "Peace Mantras" are Hindu prayers for Peace (Shanti) from the Vedas.
Generally they are recited at the beginning and end of religious rituals and discourses.

Shanti Mantras are found in Upanishads, where they are invoked in the beginning of some topics of Upanishads. They are supposed to calm the mind of reciter and environment around him/her. Reciting them is also believed to be removing any obstacles for the task being started.

Shanti Mantras always end with three utterances of word "Shanti" which means "Peace". The Reason for uttering three times is for calming and removing obstacles in
three realms which are:

"Physical" or Adhi-Bhautika,

"Divine" or Adhi-Daivika

and "Internal" or Adhyaatmika

According to the scriptures of Hinduism sources of obstacles and troubles are in these three realms.

Physical or Adhi-Bhautika realm can be source of troubles/obstacles coming from external world, such as from wild animals, people, natural calamities etc.

Divine or Adhi-Daivika realm can be source of troubles/obstacles coming from
extra-sensory world of spirits, ghosts, deities, demigods/angels etc.

Internal or Adhyaatmika realm is source of troubles/obstacles arising out of ones own body and mind, such as pain, diseases, laziness, absent-mindedness etc.

These are called "Tapa-Traya" or three classes of troubles.
When Shanti mantras are recited, obstacles from these realms are believed to be
pacified.

OM may my organs, speech, Prana, eyes and ears be nourished and be well, so should be all (my) senses become strong. Upanishad says all this (world) is Brahman. I don't reject Brahman, may Brahma not reject me. Let there be no rejection, let there be no rejection at all in me, let me be concentrated on Self, all those ways of righteous living told in Upanishads be in me! Be in me, OM Peace, Peace, Peace.

OM let my speech be established in (my) mind, and (my) mind be established in my speech, let Brahman (Supreme Reality) reveal itself to me, may (I) be able to grasp the truths of the Vedas, let not what I have heard (studied) forsake me, let me spend both day and night in study, I say rta ( divine law ), I say truth, may that (truth) protect me, may that (truth) protect teacher, may it protect me, may it protect teacher,
OM Peace, Peace, Peace.

OM O gods! Let us hear good (auspicious) things from (our) ears, O worshipful ones! let us see good (auspicious) things from (our) eyes, let (my) organs and body be stable, healthy and strong, let us do what is pleasurable to gods in the life span allotted.